1. Field of the Invention
The present invention relates generally to the restoration of teeth, and more particularly, to methods and devices for improving the accuracy and simplifying the process of performing such restorations.
2. State of the Art
Presently, numerous methods exist for the restoration of teeth by dentists, including the use of artificial tooth material (such as gold or porcelain) to form a cast-restoration or a metal-ceramic restoration (i.e., prosthetic crowns). Prosthetic crowns are typically used to repair decayed tooth structure where support from the original tooth structure is either marginal, or unavailable.
Known techniques for preparing a tooth to receive a crown are susceptible to numerous variables, some of which are within the dentist's control and some of which are not; however all of these variables can detrimentally influence the accuracy with which: (1) the tooth is prepared to receive the crown; (2) the crown is prepared for placement on the tooth; and (3) the manner by which the crown is fit to and fixed on the prepared tooth.
For example, in preparing the original tooth to receive the crown, a dentist will typically use various shaped diamond burs in a high speed hand tool to remove approximately 2 mm of exterior tooth structure. The coronal portion of the tooth is shaped so that when the prosthetic crown is received from the laboratory, it will be of the approximate size and shape of the patient's original coronal portion prior to preparation. The skill of the dentist in manipulating the tool is critical. The dentist must accurately shape the tooth (e.g., retain tooth pulp) to ensure vitality. Further, the dentist must be careful to shape sidewalls of the tooth in a manner which will maximize retention; that is, only necessary portions of the tooth should be removed.
Because tooth preparation is performed totally by manipulation of a hand tool, and because the skill required for such tooth preparation will vary among dentists, the precision with which a tooth is prepared will vary widely. In some cases, too much tooth will be removed, thereby reducing retention or destroying vitality. In other cases, too little tooth will be removed, thereby requiring that the entire procedure be repeated, including preparation of a new crown.
The artistic ability of the dentist also plays a significant role in preparing the crown. That is, the prosthetic crown used to replace original tooth structure will vary in quality based on the skill on the dentist. Further, the quality of the prosthetic crown will vary based on the skill of the person who actually produces the crown (e.g., laboratory technician).
More particularly, after the patient's tooth has been shaped to receive the prosthetic crown, an accurate impression is formed from the prepared tooth. That is, an impression material is placed into the patient's mouth to form a negative impression of the prepared tooth. To accurately prepare the impression, all gingival bleeding must be stopped and the margin of the gum tissue must be retracted from the lower portion of the tooth. The impression material must then be properly injected into the sulcus area of the tooth. A tray which contains a combination of impression materials is then applied with pressure over the teeth in the area of the prepared tooth, including the prepared tooth.
Despite efforts by the dentist to obtain an accurate impression of the prepared tooth, many factors can detrimentally influence quality of the impression. For example, the characteristics of impression material vary widely. Further, the ability of the dentist to maintain a dry field of operation in the area of the prepared tooth can inhibit accuracy of the impression. The retraction of the gingival tissue can also effect the accuracy of the impression, as can the dentist's technique in obtaining the impression (i.e., the general care in obtaining an accurate impression).
Thus, the dentist's skill plays a significant role in accurate tooth restoration, both in preparing the tooth structure to receive the crown, and in obtaining the impression used to form the crown. While the precision with which these tasks are performed is largely within the dentist's control, many aspects of tooth restoration are not. For example, no matter how skilled the dentist, there are limits to the precision with which sidewalls of the tooth can be prepared. Ideally, to optimize retention, the sidewalls of the tooth should be parallel (i.e., orthogonal to the base of the tooth). However, even the most skilled dentist will prepare the tooth with sidewalls that are sloped on the order of fifteen (15) degrees, thereby removing excess tooth structure. Further, despite the talents of an extraordinarily skilled dentist, crown preparation is typically performed by a laboratory technician using the impression prepared by the dentist, and variables extant during crown preparation can degrade the quality and fit of the crown.
The typical laboratory procedure for crown preparation is as follows. Once the laboratory receives the impression from the dentist, the laboratory technician will set die pins in the impression and then form a master impression as a die (e.g., plaster models) of the patient's teeth. The technician will set the occlusal bite registration and articulate the models of the patient's teeth. Afterwards, the laboratory technician will saw the die to remove the tooth of interest, then trim the die of the tooth and mark the marginal finish line. The sub-structure is then waxed for preparation of the prosthetic crown.
After a wax pattern has been formed, it is converted into a casting (e.g., metal casting) to serve as a sub-structure (e.g., coping) of the crown. It is a challenge to produce a casting that will comply with acceptable tolerances, given the variables associated with the quality of the impression, the skill of the technician and the proper selection of die materials. Assuming that a satisfactory wax pattern has been prepared, the mold must be enlarged uniformly using known techniques of spruing and investing the wax pattern. After the investment is built up and hardened, the wax is burned out, followed by a complex casting technique necessary to prepare the sub-structure. Assuming accurate preparation of the casting alloy, the sub-structure is divested, sprue is removed and the finished sub-structure casting is prepared.
The casting is sand-blasted and steam-cleaned. The framework of the casting is degassed and an opaque primary coating is applied. A secondary opaque coating is applied followed by a porcelain build-up, with the build-up incorporating specific shading and color effects to assimilate the enamel of the original tooth. The porcelain build-up is then vacuum fired.
The combination of the cast sub-structure and porcelain build-up constitute the prosthetic crown. The final stages of crown preparation include a finishing of the porcelain build-up, after which the anatomy of the original tooth structure is carved therein. The porcelain crown is then glazed, and the cast interior of the crown is sand-blasted to remove external oxidation. The metal interior is then polished and the fit, shading and prosthetics of the crown are quality checked. The finished crown is then returned to the dentist for placement onto the prepared tooth structure.
The process of shipping the impression from the dentist to the laboratory technician, the preparation of the crown and the returning of the crown to the dentist typically involves a period of approximately two weeks. Upon receipt of the prosthetic crown from the laboratory, the dentist removes a temporary crown which had been placed over the prepared tooth of the patient following preparation of the impression. The permanent crown is then cemented into place. The dentist's skill is again called upon to ensure proper fit, occlusion bite registration and aesthetics of the prosthetic crown. While the dentist can modify the occlusion of the crown, inaccuracies in fit can require that a new crown be prepared and the entire process described above repeated, thus leading to increased time delays and patient discomfort (e.g., due to prolonged use of a temporary crown). In some cases, if the crown does not accurately fit, the dentist will use a bur to ground the interior; however, the use of a bur to shape the crown interior alters the fit and therefore detrimentally affects the marginal seal.
Conventional techniques for tooth restoration are described in the "Textbook of Operative Dentistry", Second Edition by Lloyd Baum et al, W. B. Saunders Company: 1985, Philadelphia, Pa. Further, conventional techniques for tooth preparation in conjunction with tooth restoration are described in the document entitled "Tooth Preparation in Fixed Prosthesis (Part I)" by Arnold S. Weisgold DDS et al, The Compendium of Continuing Education, Vol. I, No. 6, November/December 1980, pages 375-382 and pages 35-41, from the General Restorative Dentistry II, 1992, Course Component: Fixed Prosthodontics, Dr. Harold Baumgarten, et al.
In summary, conventional techniques for tooth restoration using prosthetic crowns are complex and are susceptible to substantial inaccuracies that stem from the skill level of the dentist and laboratory technician. These complexities and skill-dependent tasks translate into patient discomfort (due to improper fit) and increased costs. Further, the potential inaccuracies in tooth restoration can lead to crowns of relatively short life.
Thus, it would be desirable to improve the accuracy with which tooth restorations are performed. Further, it would be desirable to reduce the skill-dependent tasks associated with tooth restoration, and to reduce the cost associated with such procedures, without compromising the quality of these procedures.